Carbon fiber reinforced plastic, hereinafter abbreviated as “CFRP”, is lightweight, highly rigid, and highly corrosion resistant. Attempts have been made to adapt CFRP to outer panels of various industrial machines and transportation apparatuses such as automobiles. For example, a type of CFRP called a “sheet molding compound” (SMC) is widely used in outer panels of automobiles, such as hoods and fenders.
A sheet molding compound disclosed in Japanese Unexamined Patent Application Publication No. 6-286008 is a slurry intermediate material containing reinforcing fibers, which are short glass fibers several centimeters in length, and a polyester resin or the like. The SMC is heated in a mold and shaped by pressing at a high pressure of 50 or more atmosphere to form a base panel for the outer panel. The surface of the base panel is worked with abrasive paper, a file, or the like so as to make the surface flat and smooth. Subsequently, a color coat is applied to make a CFRP outer panel for automobiles, for example.
Outer panels composed of SMC comprising short reinforcing fibers (discontinuous fibers) have a stiffness lower than that comprising continuous reinforcing fibers. This is not only because the reinforcing fibers are short but also because the elasticity of glass is 70 GPa, which is only one third of the elasticity of steel, which is 210 GPa. Therefore, the thickness of the SMC outer panels is larger than that of metal outer panels. Weight-reduction is thus not necessarily achieved in comparison with metal outer panels, and if there is any weight-reducing effect, it is often very small. Moreover, since fibers of SMC outer panels are discontinuous, SMC outer panels break at the discontinuous portions and thus cannot exhibit high vibration-resistant characteristics, i.e., tensile fatigue/bending fatigue characteristics. In other words, the number of repetitions until breaking under a particular average stress is small. Furthermore, SMC outer panels may be easily perforated and damaged by a local impact such as that caused when an object, such as a rock, flies and hits the SMC outer panel. Accordingly, anti-impact protection measures, such as increasing the thickness or affixing rubber, must be taken for outdoor-use outer panels such as those of transportation apparatuses. Thus, lightweight outer panels that can replace metal outer panels, i.e., environment-friendly automobile outer panels, have not been realized with SMC.
Use of continuous fibers improves the impact-resistance. In particular, use of a continuous-fiber woven fabric having a net structure significantly improves the impact-resistant characteristics. However, since the woven fabric used is heterogeneous and anisotropic, local differences in temperature and moisture absorption over time occur, resulting in generation of microcracks around the surface.
As described above, CFRP outer panels using continuous fiber woven fabrics have many problems and their practical application is few. No reference marks that quantitatively indicate the effect of the surface quality of practical CFRP panels on the structure and the durability have been established to date.
Each company has its own proprietary standard, such as that using a glossmeter or a tension meter, for the surfaces of metal outer panels based on its long experience. However, CFRP has mechanical properties, a hardness, and a linear coefficient of expansion different from those of metal, and the practical standard for the surface is different from that of metal. Simply applying the standard for metal does not provide CFRP outer panels that can withstand practical use.
For example, when a tool or the like drops onto a metal outer panel during production, a dent (permanent deformation) is formed by plastic deformation. The quality of the surface can be easily determined by observation with the naked eye. However, CFRP does not undergo plastic deformation. No dent that recognizable by the naked eye is formed, but separation occurs inside.
Accordingly, examination of dents on the surface using the same standard as that of metal results in overlooking of separation inside the CFRP. The internal separation becomes worse in long-term use, and the quality required for outer panels can be no longer maintained. Moreover, water gathers in the separated portions, thereby increasing the weight. As the temperature rises, evaporation of water pushes out the coating film from the inside, resulting in bulging of the coating film.
The surface quality of outer panels is known to significantly affect not only the long-term durability and merchantability but also the drag with respect to air or water. For the purpose of energy conservation, not only automobiles but also transportation apparatuses in general, such as trains, small aircrafts, boats, and ships, require improvements in surface quality. Generally, when a CFRP outer panel is used to achieve weight reduction, the CFRP outer panel is significantly deformed by air pressure applied during traveling of the transportation apparatus at high speed since the elasticity is lower than that of metal. This results in a large change in drag. In view of the above, the surface of the CFRP should be designed differently from metal materials.
In order to practically apply CFRP outer panels using continuous fibers, a comprehensive technology that is suitable for CFRP outer panels and that can quantitatively indicate the effect of the surface quality on the structure and the durability must be established.